In general, a differential gear apparatus of this type comprises an internal gear and a sun gear rotatably arranged with their axes aligned with each other, a carrier rotatably arranged with its axis aligned with the axes of the internal gear and the sun gear, and a planetary gear rotatably (rotatably about its own axis) supported by the carrier and meshed with the internal gear and the sun gear. When the carrier is driven for rotation, its rotation is transmitted to the internal gear and the sun gear through the planetary gear. When the planetary gear is not rotated about its own axis, the internal gear and the sun gear are integrally rotated together at the same rotational speed and without being rotated relative to each other. On the other hand, when the planetary gear is rotated about its own axis, the internal gear and the sun gear are differentially rotated in accordance with rotational speed of the planetary gear (see Official Gazette of Japanese Patent Application Laid-Open No. H09-112657).
In the above-mentioned conventional differential gear apparatus, there is such a problem that when the respective pitch circle diameters of the internal gear and the sun gear are determined, the ratio of rotation torques (torque bias ratio) transmitted to the internal gear and the sun gear is primarily determined and the ratio is unchangeable. For example, in case the respective circle pitch diameters of the internal gear and the sun gear are represented by D1, D2, respectively, the torque bias ratios are primarily determined as D1: D2.